Fluid flow induced by nonuniform ac electric fields in electrolytes on microelectrodes. I. Experimental measurements

Abstract

Under the influence of an ac electric field, electrolytes on planar microelectrodes exhibit fluid flow. The nonuniform electric field generated by the electrodes interacts with the suspending fluid through a number of mechanisms, giving rise to body forces and fluid flow. This paper presents the detailed experimental measurements of the velocity of fluid flow on microelectrodes at frequencies below the charge relaxation frequency of the electrolyte. The velocity of latex tracer particles was measured as a function of applied signal frequency and potential, electrolyte conductivity, and position on the electrode surface. The data are discussed in terms of a linear model of ac electroosmosis: the interaction of the nonuniform ac field and the induced electrical double layer.

Pumping of liquids with traveling-wave electroosmosis

“…If the ac electric field is nonuniform, the component of the ac field normal to the electrode surface induces charge in the double layer, while the tangential component produces a body force on the induced charge, thus moving the fluid. [9][10][11][12][13][14] The generated velocity field has a steady-state component which is observed experimentally. Each asymmetric pair of electrodes produces a nonsymmetric local flow that eventually generates a global flow in the direction of broken symmetry.…”

“…Fluid flow due to ac electroosmosis is observed in the region of this characteristic frequency c . 8,9,11,13 Ehrlich and Melcher studied theoretically the fluid motion induced by traveling wave in a semi-insulating bipolar liquid. 16 As in the present work, they took into account the charge induced in the double layer.…”

“…The charging of the double layer is analogous to the charging of a capacitor ͑the double layer͒ through a resistor ͑the ohmic a͒ Author to whom correspondence should be addressed; electronic mail:ramos@us.es liquid͒. 9,[11][12][13] The charging time is therefore of order of c = ͑ / ͒͑ᐉ / D ͒, where ᐉ is a typical length of the system and / D is the surface capacitance of the double layer. When the frequency of the applied signal is low c Ӷ 1, most of the applied voltage is dropped across the double layer, and the tangential electric field is small.…”

Numerical study of dc-biased ac-electrokinetic flow over symmetrical electrodes

“…7,8 AC-electrokinetic (ACEK) makes use of AC electric fields acting on AC charge density to induce rectified flow; two important examples of ACEK are AC-electroosmotic (ACEO) and AC-electrothermal (ACET) flows. [9][10][11][12][13][14][15][16] ACEO flow is based on the Coulomb force acting on the induced charge in the double layer in the presence of a tangential AC electric field. [10][11][12][13] ACET flow is based on the interaction of AC electric fields with conductivity gradients in the fluid induced by a thermal gradient.…”

“…[9][10][11][12][13][14][15][16] ACEO flow is based on the Coulomb force acting on the induced charge in the double layer in the presence of a tangential AC electric field. [10][11][12][13] ACET flow is based on the interaction of AC electric fields with conductivity gradients in the fluid induced by a thermal gradient. [14][15][16] More recently, a DC-bias AC voltage (V applied ¼ V DC þ V AC cos xt) has been employed effectively to concentrate particles/cells [17][18][19] and for pumping/mixing [20][21][22][23] applications in microfluidics.…”